Automatic gain control circuit



May 22, 1962 A.` T. BROWN nl AUTOMATIC GAIN CONTROL CIRCUIT Filed Juneze, 195s Inventor ALEX T. ,eww/UZ! By lulc MLU Agent 3,036,276 AUTMATICGAIN CONTRGL CIRCUT Alex T. Brown m, Wayne, NJ., assigner toInternational Telephone and Telegraph Corporation, Nntlcy, NJ., acorporation of Maryland Filed .lune 26, 1958, Ser. No. 744,702 14Claims. (Cl. S30-138) This invention relates to automatic gain controlcircuits and more particularly to amplified delayed automatic gaincontrol circuits.

The term delayed automatic gain control (AGC) circuit employed herein isdefined as an AGC circuit which enables an amplifier system to operateat a given gain until the signal passing through the amplifier systemeX- ceeds a predetermined level at which time the AGC circuit willfunction in a manner to reduce the gain of the ampliiier system tomaintain a signal output from the amplier system at substantially saidpredetermined level.

An object of this invention is to provide a novel amplified delayed AGCcircuit employing a transistor amplifier.

Another object of this invention is to provide a transistor amplifier ina delayed AGC circuit providing a very close control of the amplitude ofthe output signal from the controlled amplifier system once said outputsignal has achieved a predetermined amplitude.

Still another object of this invention is the provision of atransistorized amplified delayed AGC circuit to control the gain of aplurality of amplifier systems equally.

A feature .of this invention is to provision of a transistor amplifierto amplify an AGC control signal indicative of the amplitude of theoutput signal of the amplifier system for application to said amplifiersystem to control the gain thereof and a voltage source coupled to saidtransistor amplier to render said transistor non-conductive below agiven level of said control signal and conductive above said givenlevel.

Another feature of this invention is the provision of a voltage sourcehaving a given value of voltage coupled to the emitter electrode of thetransistor amplifier employed in the delayed AGC circuit of thisinvention to render the transistor amplifier non-conductive until suchtime that the AGC control signal and hence the amplifier system outputsignal exceeds said given value of voltage.

Still another feature of this invention is the provision of a delayedAGC circuit enabling the operation of the amplifier system beingcontrolled at full gain until such time as the AGC control signalexceeds the bias voltage applied to the transistor amplifier in thedelayed AGC circuit of this invention at which time the delayed AGCcircuit will function to reduce the gain of the amplifier system tomaintain a signal output of the amplifier system at substantially thesignal level which overcomes said bias voltage.

A further feature of this invention is the provision of a transistorizedampliiied delayd AGC circuit which may be modified to operate witheither a positive or negative voltage source coupled to the emitterelectrode of the transistor amplifier to establish a conduction levelfor said transistor amplilier.

The foregoing and other objects and features of this invention and themanner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram partially in block form of a diversityreceiving system incorporating one form of the automatic gain controlcircuit following the principles of this invention; and

FIG. 2 is a schematic diagram of another form of automatic gain controlcircuit in accordance with the principles of this invention which may besubstituted for that 3,36276 Patented May 22, i962 portion of the systemof FIG. 1 to the right of the dashdot line A-A.

The AGC circuit of this invention will be described hereinbelow in anoperable combination with a diversity receiving system of the equal gaincombining type. It is to be remembered, however, that the AGC circuit ofthis invention is not restricted to its use with a diversity receivingsystem. The AGC circuit of this invention may -be employed to controlthe gain of any amplifier system which has the requirement of thecircuit hereindescribed, that of delayed AGC.

Referring to FIG. l, one form of the delayed AGC circuit l of thisinvention is illustrated in an cooperative operative arrangement with anequal gain combining diversity receiver. Briefly, the equal gaincombining receiver includes two signal channels 2 and 3. Each of thesignal channels includes an antenna 4, an RF amplifier 5 to select andamplify the signal induced in antenna 4,

amplifiers 8. The outputs from IF amplifiers 8 and 8a are then combinedin combiner 9 to provide diversity advantage in the resultant combinedsignal which is coupled along conductor lt) to the remainder of thediversity receiving system. T o achieve the desired diversity advantagein this type of diversity system it is a requirement that the outputs ofIF amplifiers 8 and 8a are combined in-phase. To assure the proper phaserelationships between the signals at the outputs of IF arnpliliers 8 and8a for in-phase combining inthe combiner 9, a phase detector circuit isincluded in combiner 9 to `detect any phase difference between thedesired phase relationship of the outputs of amplifiers 8 and 8a. Thedetection of a phase difference between the phase relation of these twosignals produces the phase control signal which is conducted along phasecontrol bus 11 to at least `one of the oscillators 7. This phase controlsignal will operate upon at least one of oscillator 7 to adjust therelative phase relationship between the outputs of the mixers 6 and-hence the outputs of amplifiers 8. `In this manner the predeterminedphase relationship between these two signals is maintained so that thesetwo signals may be combined substantially inphase in combiner 9. Adetailed description of the circuitry which may be included in combiner9 is disclosed in the co-pending application of R. T. Adams, entitled,Combining System for Diversity Communication System, Serial No. 737,122,filed May 22, 1958. The manner of obtaining the phase control oradjustment of the output of oscillator 7 may be accomplished by applyingthe phase control signal to a reactance tube in coupled relation withthe oscillator '7 or may be accomplished in the manner described indetail in the co-pending application Eof R. T. Adams, entitled,Oscillator Circuit, Serial No 737,754, filed May 26, 1958.

The overall receiving system thusfar briefly described is disclosed indetail in the co-pending application of F. I. Altman, A. T. Brownentitled Radio Diversity Receiving System, Serial No. 26,817, filed May4, 1960, a continuation-in-part of Serial No. 719,181, tiled February27, 1958, now abandoned. In this co-pending application it is pointedout that the overall receiving system may be employed for either spacediversity wherein the antennas 4 are separated by a given distance toachieve a diversity advantage operating at the same radio frequency orthe I the AGC circuit of thisl invention which is of the delayedytypeandidentified inFlG; 1,' :by the reference character 1, is`incorporated to provide the desired equal gain in IF amplitiersjS and8a. p Y

The delayed AGCcircuit 1 includesV an amplitude or AGC'detector 12' todetect the amplitude of the signal at theoutputA of combiner 9 toproduce a control signal which is vjgiroportionaltothe amplitude of thesignal output from combiner 9. In the arrangement illustrated in'FIG. 1the negativeside or reference' potential side of the AGCdetector,indicated by the minus Ysign 'and reference'character 13, is connecetdto a point'on voltage divider 14 toremove the reference potential of theAGC detector fromground; In this embodiment Voltage divider 14 iscoupled betweenground and a voltage source coupled at terminalltn havingnegative polarity. Conductor 13 is then coupled to point ln voltageVdivider 14 to provide the desiredv negative reference' potential forVAGC detector 12.

The Ycontrol Ysignal output from 'the AGC detector 12 of 'transistorVamplifier 17. The emitter'electrode 18 of th'etransistor amplifier 17is coupledrthrough a current lin'iitin'gresistor 19 to a potentiometer20 included as Ya Ifshuld beV of sufficient valuetorbias transistoramplifier 17 to'non-conduction until such time as the control signalfrom the AGC detector 12 vand hence the# output 'signal from? combiner 9'exceeds this level set voltage. VAt the Vtime that the amplitudeY ofthe output signal y.from combiner 9 exceeds the level set voltage or in'other Words whenfa 'control output of the AGC de-v Y Vis"'coupl'edfalong conductor 15 to the'base electrode 156 tecto'ulz exceeds thislevel set voltage Vthe transistor 7' amplifier 17 Wil-l begin toconduct. The collector current thatis produced from collectorelectrode'ZS at the time of conduction of transistor amplifier 17 isthen coupledV along AGC bus Mito control the gain of the amplifiers 3.The amplified AGC control signal is coupled alongfthe Y AGC bus24'in thesystem illustratedV herein to a pair of Y potentiometers 254 and 26;from which a given amountV of the amplified AGC control signal iscoupled to Ypreferably the control grids of IF amplifiers 8 and 8a,respectively, *to

provide equal gain in these amplifiers to thereby enable the equal 'gaincombining'of theV diversity signals. The actionof the amplified AGCsignal coupled through AGC bus 24 andpotentiometers 25 and 26 is-tolower the AGCV the output of combiner 9 to buck out the delay or levelsetvoltage. A very small change in base-emitter voltagecauses the basecur-rent to change sufficiently to give a large change in the AGC busvoltage thereby producing a Very close control of the IF output at theoutput of combiner 9 approximately inthe Vicinity of the amplitudeestablished by the level set voltage.Y

In summary it can 2be stated that the AGC circuit `1 of this inventionincludes a means (AGC vdetector 12) to detect the amplitude of a signalat the output of an amplifier system and producea control signalproportional to this amplitude, a Vtransistor' amplifier 7 coupled tothe output of the detector means to amplify Vthe control signal,andmeans coupled to transistor amplifier 17 to couple the amplifiedcontrol signal to the amplifier system to control the gain of thisamplifier system, randa voltage source coupled tothe transistor-amplifier 17 to render transistor amplifier 17 non-conductive -below agiven level of the output ofthe lamplifier system and conductive aboutthis given level. The action of this circuit is to provide a relativelyclose'control of the output of an `amplifier' system to provide an`output 'signal which is .held Vrelatively close to thebias or level setvoltage biasing transistor amplifier 17.

VVFor optimum diversityV adding in combiner 9, the gain characteristicsof the two amplifiers S `and 8a Vmust track relatively close. Toaccomplish this result, attennatorsy 27 and 27a at the inputs of amplicr8 and 8a are adjusted so that the gain of these amplifiers coincide atzero AGC control signal. lfarnplier 8a has a higher gain than amplifier8, this preferably is accomplished by'maintaining attenuator 27 at Zeroattenuation and inserting attenuation in attenuator 27a to reduce thegain of amplitier 18a to coincide with the gainv of amplifier 8. Thetrackingof the gain of amplifiers S and Sa'and'the amount of VAGC signalcoupled to these 'two' amplifiers to maintain substantially'equal gainin these two amplifiers is then provided by an appropriate adjustment'ofpotentiometers 25 and26. Through the employment of thesetwopotentiomete'rs 2S and 26 it is possible to adjust the slope of gaincharacteristics of-arnplifiers 8 and 8a such that whenlthe AGC circuitis in operation the gain characteristicV ofV these two-amplifiers trackeach other very closely .over itherwhole rangeof gain control and henceVprovide the desired equal gain characteristics in arn- ,combiner` .9.Preferably, the slope of Vthegain characteristics of-onlyone amplitierischanged. VThis is accom- Vcharacteristic of the otherampliiier.

plished by fixing one of the potentiometers to pass all of the controlsignal and adjusting the other potentiometer to make the slope of thegain characteristics of its associated amplifier coincideV with theslope of the gain Ina successfulV reduction to practice, I have employedthe `followingfcomponents' and component values for the' AGC circuitillustrated in llG. l.- The following is a llistpof such components;

v 'Transistoramplifier 17 2N78Transistor.

bias ongth'e control grids of amplifiers 'Y Sand 3a toanegative value. LY' o In operation Vwhen the signal at the outputzcornbinerl 9 is belowthe Vpredetermined signal level-set bythe level setV voltage produced involtage divider 14 thetransistor r jamplifier 17 is renderednon-conductive thereby providingV Y f zero voltage-on theAGCbusrgiving.maXimumRIF gainV 'j iti-the amplifiers Sand Sci. As thecombiner Sf'output-V `andfliencethe'AGC-control ksignal approaches thevalue ofth'eilevel'set voltageV transistor amplifier 17 begins toVconduct;VY -The collector current produced by the conducf f tion ofVtransistor amplifier 17 lowers-'the AGC fbiasappliedalong.AGC-bust'tofanegative valu'e. A Vbalance 'reachedwhen the AGCwbias sets `the properamplifier gain to maintainpa counteracting voltagefromcontrollcircuit1in -ajsingle channel receiver or other Y* `Sl/ SriisrequiringAGC to Vfc'ontrrol'thef gain of a singleY Y Y' Employing theabove listed component values,- the AGC Y jcircuit'of'IlG. l hassuccessfully controlled the gains of vamplifiers 8 and SaasV describedhereinabove. fIt is also possiblerasV pointed out hereinabove to employthe gain lamplier ory amplifier system.kr

Referring to FG. 2 there is shown another form of AGC circuit 1 of thisinvention. The circuit of FIG. 2 operates substantially in the samemanner as described in connection with FIG. 1 and includes the samecomponents such as transistor amplifier 17 and voltage divider 14 toprovide the desired level set voltage to bias amplifier 17 intonon-conduction when the signal output of AGC detector l2 is below agiven value set by the level set voltage of voltage divider 14. Once thecontrol signal exceeds this level set voltage applied to emitterelectrode 18, the tx-ansistor amplifier l? will function as describedhereinabove to control the gain of the amplifier circuits 8 and 8a toprovide substantially equal gain therein and to maintain the output ofcornbiner 9 at substantially the level set by the level set voltage. Thedifference between the circuit of FIG. 2 and the circuit of FIG. l isthe bias voltage produced in voltage divider 14 and the connection ofthe potentiometers 25a and 25a. To terminal Mb is applied a positivevoltage to be divided in voltage divider 14 to provide the desired biasor level set voltage coupled to bias emitter electrode 18. in thisparticular circuit the reference potential conductor 13 coming from theAGC detector 12 is coupled to ground reference potential rather than anegative reference potential as is present in the circuit illustrated inFIG. l. In the embodiment illustrated in FlG. 2 as well as theembodiment illustrated in FIG. l the emitter electrode is maintainedpositive with respect to a base electrode by the voltage applied fromvoltage divider 14 and hence maintains transistor amplifier 17non-conductive until the input along conductor i from AGC detector 12exceeds this given value at which time the transistor amplifier 17 Wil-lfunction to control the gain characteristics of the Iamplifiers S and8a. The amplified control signal on conductor "24 is coupled topotentiometers 25a and 26a as in FIG. l. However, rather than having thepotentiometers returned to ground, the potentiometers are returned toterminal 1417.

In operation, before the AGC circuit is connected into the system, theamplifiers S and 8a are adjusted to be biased to substantially cut-offwhen the grids are grounded. The transistor amplifier 17 and itsassociated circuits is then connected into the system and hence returnthe grids of ampliers 8 and 8a to a positive potential. By virtue of thefact 4that potentiometers 25a and 26a are returned to terminal 14b,amplifiers S :and da operate Yat full gain when transistor amplifier 17is non-conductive. As soon as transistor amplifier 17 is renderedconductive, the bias applied to amplifiers S and 8a is reduced, mademore negative, and hence reduces the gain of amplifiers 8 and 8a.

While terminal 14h is shown as having a separate voltage source appliedthereto, this could be connected to the cathode of the amplifier beingcontrolled and, hence, the voltage which renders the amplifier cut-offbefore the AGC circuit operates is connected into the amplifier system,This arrangement would perform best to control the gain or single stageamplifier.

The transistor amplifier employed in AGC circuit l of both FIGS. l and 2is illustrated to be NPN type transistors. It should be obvious to oneskilled in the art that PNP type transistors could be substituted forthose illustrated in the circuit of FIGS. l and 2 with the appropriateadjustments in the bias voltage delivered from the voltage divider tobias the transistor amplifier nonconductive below a given level andconductive above a given level. The bias level should be such, whenemploying a PNP type transistor, to render the emitter electrodenegative with respect to the base electrode. With this change theoperation of the AGC circuit employing the PNP type transistor will bethe same as described hereinabove with respect to the NPN typetransistor amplifier.

While l have described above the principles of my 6 invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by Way of example and not as a limitationto the scope of my invention as set Iforth in the objects thereof and inthe accompanying claims.

I claim:

l. An amplified automatic gain control circuit operable to control thegain of a plurality of amplifier systems equally, each of said amplifiersystems having an input terminal coupled to a source of signal to beamplified, an output terminal, and a gain control terminal comprising ameans to combine the signals at the output terminals of said amplifiersystems substantially in-phase to produce a single signal therefrom,means coupled to said combining means to detect the amplitude of thesignal at the output of said combining means and to produce a controlsignal proportional to said amplitude, a transistor amplifier coupled tothe output of said detector means to amplify said control signal, meanscoupled to said transistor amplifier to couple said amplified controlsignal to the gain control terminal of each of said amplifier systems tocontrol the gain of each of said amplifier systems equally in accordancewith said amplified control signal and a bias source coupled to saidtransistor amplifier to render said transistor amplifier non-conductivebelow a given level of said control signal and conductive thereabove.

2. An amplified automatic gain control circuit operable to control thegain of a plurality of amplifier systems equally, each of said amplifiersystems having an input terminal coupled to a source of signal to beamplified, an output terminal, and a gain control terminal comprising ameans to combine the signals at the output terminals of said amplifiersystems substantially in-pnase to produce a single signal therefrom,means coupled to said combining means to detect the amplitude of thesignal at the output of said combining means and to produce a controlsignal proportional to said amplitude, a transistor amplifier to amplifysaid control signal including at least three electrodes, means couplingthe output of said detector to one of said electrodes, means couplinganother of said electrodes to the gain control terminal of each of saidamplifier systems to control the gain of each of said amplifier systemsequally in accordance with said amplified control signal, saidlast-mentioned means including a plurality of potentiometers, one ofsaid potentiometers being coupled to each of said gain control terminalsto equalize the gain characteristics of said amplifier systems and aconductor coupled to said another of said electrodes and each of saidpotentiometers to supply said amplified control signal thereto forapplication to their respective ones of the gain control terminals ofsaid amplifier systems, a voltage source having a given value of voltageand means coupling still another of said electrodes to said voltagesource to render said transistor amplifier non-conductive until saidcontrol signal exceeds said given value of voltage.

3. An amplified automatic gain control circuit operable to control thegain of a plurality of amplifier systems equally, each of said amplifiersystems having an input terminal coupled to a source of signal to beamplified, an output terminal, and a gain control terminal comprisn Ving a means to combine the signals at the output terminals of saidamplifier systems substantially in-phase to produce a single signaltherefrom, means coupled to said combining means to detect the amplitudeof the signal at the output of said combining means and to produce acontrol signal proportional to said amplitude, a transistor amplifier toamplify said control signal including at least three electrodes, meanscoupling the outputV of said detector to one of said electrodes, meanscoupling another of said electrodes to the gain control terminal of eachof said amplifier systems to control the gain of each of said ampliersystems equally in accordance Vwith said amplified control signal, saidlast-mentioned means including a plurality of potentiometers, one ofsaid potentiometers being coupled to each of said gain control terminalsto equalize the gain characteristics of said amplifier systems and ayconductor coupled-to said another of said electrodes and each of saidpotentiometers to supply said amplied control signal thereto forapplication to their respective ones of said gain control terminals, avoltage source, a voltageY divider coupled between said voltage sourceand ground potential including a movable tap disposed for movement alonga given portion of Vsaid voltage divider to4 enable the selection of agiven value of voltage and means coupling still another of saidelectrodes to said movable tap to render said transistor ampliiernon-conductive until said control signal exceeds said givenvalue ofvoltage. Y

4.` A circuit according to claimV 3, wherein the voltage ofsaid voltagesource has a negative polarity with respectto ground and means coupledbetween a point on said voltage divide and said detector establishes areference potential for said detector more negative than said givenvalue of voltage.

V5. A circuit according to claim 3, wherein the voltage kof said voltagesource has a positive polarity with respect to ground and means coupledbetween ground and said detector establishes a reference potential forVsaid detector more negative than said Ygiven value of voltage.

, 6. An lan'iplitied automatic gain control circuit operable to controlthe gain of a plurality of amplier systems equally, each of saidamplifier systems having an input terminal coupled to a source of signalto be amplitied, an output terminal, and a gain control terminalVcomprising a means to combine the signals at the output terminals ofsaid amplifier systems substantially iii-phase .to produce a singlesignal therefrom, means coupled to said combining means to detect theamplitude of the signal-at the output of said combining means and toproduce a control signal proportional to said amplitude, a v

,transistor to amplify said control signal including a base electrode, acollector electrode and an emitter electrode, `means coupling the outputYof said detector to said base electrode, means coupling said `collectorelectrode to the gain control terminal'of each of said ampliiier systemsto control the gain of each of Ysaid amplifier systems 4equally inaccordance with said amplified control signal,

' said last-mentioned Vmeans including a pluralityof po- 1 tentiometers,oneV of said potentiometers being coupled toY each of said gain controlterminals to equalizeV the gain characteristics of said amplifiersystems and a-conductor coupled'to said collectorY electrode and eachrofsaid potentiometers to supply said Vanipliiied control signal theretofor application to their respectiveV ones of-said gain controlterminals, avoltage Vsource having a given value ofv Voltage, meanscoupling said emitter electrode to said voltage source to render saidtransistor amplitier non-conductive until said control signal exceedssaid givenvalue of voltage.

7,-. A circuit according to claim 6, wherein thevoltage Y of saidvoltage source has a negative polarity with respect to ground andmeanscoupled between apoint on said' voltage divider and saiddetector'establishes a. referrence potential for Vsaid detectorl morenegative than said Y f given value of voltage. t e Y j 8. VA circuitaccording to-claim 6, wherein the voltage L of said-voltagesource has apositive polarity with re- Vspectto Vground and lmeans coupled betweenground. and

` said Vdetector establishes a reference potential for said t "detectormore negative than said given value of voltage.

9. An amplified automatic Vgain control circuitn operable to controlvtheV gain of, a plurality of amplifier stagesV Y n n 7G signal from saidisecondresistor, means coupling said rst comprising a detector havingtwo VVoutput. terminals,

f means coupling Lthe input of said ydetector to the output Vofsaida-mplier stages toiproduce a gain control Vsignal proportional to theamplitude oitheoutput signal therefrom, a voltage'sourc'e, .avoltagedivider coupled between dividerV to establish a referencepotential for said detector negative with respect to said given value ofvoltage, a transistor to amplify said gain control signal including abase electrode, a collector electrode, and an emitter electrode, meanscoupling said gain control signal from 10 the other terminal of saiddetector to said base electrode,

a plurality of resistors, one end of each of said resistors beingconnected to said collector electrode and the other end of each of saidresistors being connected to a second point on said voltage divider toestablish a reference po- 4tential for said resistors positive 'withrespect to said given value of voltage, a plurality of movable taps eachbeing coupledV to an associated one of said resistors, each of saidplurality of movable taps'being adjusted to couple a predeterminedamplitude of thev ampliiied gain control signal from its associated oneof said resistors, means coupling each of said plurality of movable tapsto an associated one of said amplifier stages to control the gainthereof in accordance with said predetermined amplitude of the amplifiergain control signal to equalize the gain V characteristics of saidamplifier stages, and means coupling said emitter electrodeto themovable tap of said voltage divider to render saidA transistornonconductive until said gain control signal exceeds said given value ofvoltage.

10. A circuit according to claim'9, wherein the voltage of said voltagesource has a negative polarity with respect to said ground potential,said reference potential for said detector is positive with respect tothe polarity oi tie voltage of said voltage source, and said referencepotential for said resistors is sm`d ground potential.

ll, A circuit according to claim 9, wherein the voltage of said voltagesource has a positive. polarity with respect toV said ground potential,said reference potential for said detector is said ground potential, andsaid reference potential for said resistors vis the voltage of saidvoltage source. l2. An amplified automatic;V gain control circuitoperable to control the gain `of rst and second amplifier stagescomprising a detector having two output terminals, means coupling theinput of said detector tothe output of said amplifier stages to produceagain control signal proportional to the amplitude of the output signaltherefrom, a Voltage source, a voltage divider coupled between saidvoltage source and ground potential including a movable tap disposedVfor movement along a given portion of said spect to said given value ofvoltage, a transistor to amplify said gain control signal including abase electrode, a collector electrode, and an emitter electrode, meanscoupling said gainV control Vsignal fromthe other terminal of saiddetector to said base electrode, iirst and second resistorrs, one end ofeach of said resistors being connected 'gagre, a iirst movable tapcoupled to said rst resistor to couple `a first predetermined amplitudeof the amplified gain control signal from'saidirstresistor, a secondmovable'tapcoupled to said second 'resistorv to couple a secondpredetermined amplitude vof the amplified gain Vcontrol andset'zondVamplifier stages in accordance with said rst and second predeterminedamplitude-'ofthe amplitied'gain f lrsra'id Vvoltagesoiirce and groundYpotential including afm control signal to equalizethe*V gaincharacteristics of said amplifier stages, and means coupling saidemitter electrode to the movable tap of said voltage divider to rendersaid transistor nonconductifve until said gain control signal exceedssaid given value or" voltage.

13. A circuit according to claim 12, wherein the voltage of said voltagesource has a negative polarity with respect to said ground potential,said reference potential for said detector is positive Iwith respect tothe polarity of the voltage of said voltage source, and said referencepotential for said resistors is said ground potential.

14. A circuit according to claim- 12, wherein the voltage 'of saidvoltage source has a positive polarity with respect to -said groundpotential, said reference potential for said detector is said groundpotential, and said refer-i ence potential for `said resistors is thevoltage of said voltage source.

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